Cranial perforators are special purpose drills which are used to bore holes through the skull during cranial surgery. Such holes may be needed to vent fluids from the region surrounding the brain, to provide small passageways to the brain for the insertion and removal of instruments, or to position a cranial saw for subsequent use in removing a larger piece of the skull.
Regardless of the end use of the hole being made, it is critical that the cranial perforator stop its boring action before it encounters--and thereby damages--the delicate dura tissue surrounding the brain, or the brain itself. To this end, cranial perforators have traditionally utilized a special "safety construction" designed to permit forward penetration by the perforator only so long as the perforator's leading tip is encountered hard bone, and to halt forward penetration by the perforator as soon as the perforator's leading tip passes through the hard bone and before it encounters the soft tissue beneath the bone. More particularly, cranial perforators have traditionally comprised a drill head assembly having a pair of drills disposed in concentric relation to one another, with the inner drill leading the outer drill slightly so that a bore-counterbore opening is formed as the perforator penetrates into the skull. The two drills are coupled to a rear support and drive assembly via a special clutch arrangement such that both drills are enabled so long as the leading inner drill is encountering a resistive surface (i.e., bone) and both drills are disabled as soon as the inner drill stops encountering the resistive surface (i.e., as soon as it passes through the bone). Inasmuch as the leading inner drill and trailing outer drill are adapted to cut in a bore-counterbore arrangement, the shoulder of bone formed at the intersection of the bore-counterbore opening impedes further progress of the perforator toward the brain once the leading inner drill is disabled. As a result, the surgeon using the cranial perforator can concentrate entirely on the placement of the cranial hole and need not fear that the perforator will penetrate too far into the head so as to damage the delicate dura tissue or the brain itself.
Cranial perforators using the aforementioned "safety construction" are believed to have been marketed by a number of different entitites, among them Codman & Shurtleff of Randolph, Mass. (see U.S. Pat. No. 2,842,131), Aesculap of Tuttlengen, West Germany, and a medical instrument subsidiary of the 3M Corporation of Minneapolis, Minn. Such cranial perforators have included both reusable and disposable models.
Unfortunately, the cranial perforators developed prior to this invention are believed to suffer from one or more serious deficiencies. For example, some of the prior art cranial perforators are believed to be unreliable with regard to the operation of their special "safety construction". Such unreliability is intolerable since a failure of the "safety construction" to operate as intended can have catastrophic effects. At least some prior drill-type cranial perforators of the type described above have been prone to failure or to unreliable operation of their "safety construction" under non-axial loading. Because the prior safety construction usually involves a pair of diametrically-opposed members coupling the drill head assembly to the rear support and drive assembly, non-axial loading of the drill head assembly can give rise to a chattering action of sufficient magnitude to prematurely terminate the useful life of the safety construction of the perforator or to cause the safety construction to operate unreliably, e.g. possibly by intermittently disengaging the drill head assembly from the rear support and drive assembly. Other problems with prior art devices involve difficulty of manufacturing component parts with necessary precision, so that both quality and cost tend to suffer. Furthermore, it appears that virtually all or many prior art cranial perforators have proven difficult to center when starting a bore. This is particularly true when the bore site is well lubricated by blood or other fluids, so that the perforator has a tendency to skate or slip along the outside of the skull. This can result in unnecessary added injury to the patient. Also, at least some prior art cranial perforators tend to render unusable the cranial material removed from the bore site, with the result that the holes made by such perforators may have to be refilled with foreign materials at the close of surgery rather than with the patient's own bone matter. Also, most, if not all, prior art cranial perforators have required relatively high speed drilling (i.e., drilling at speeds of around 800-1000 rpm), which tends to be less desirable than low-speed drilling (i.e., drilling at speeds of around 100 RPM) for a variety of reasons.
In addition to the foregoing, in some prior art reusable cranial perforators, the rear support and drive assembly and/or the clutch means are inadequately designed to withstand repetitive non-axial loading, with the result that they fail after an indefinite period of use. Also, many prior art reusable cranial perforators are designed so that they must be disassembled into numerous pieces to effect proper cleaning, with the result that disassembly and subsequent reassembly tend to be fairly complicated and time-consuming. Moreover, with many prior art designs, if reassembly is improperly effected, the perforator's special "safety construction" may be rendered totally inoperative, in which case the perforator's forward penetration will not be halted automatically as soon as its leading tip passes through the skull, and damage to delicate tissues can ensue unless the surgeon is skillful and observant enough to prevent penetration into the dura.
In addition to the foregoing, disposable cranial perforators heretofore known have generally lacked safeguards to ensure that the perforator cannot be reused.
Cranial perforators making use of inner and outer drills have, on occasion, lacerated or damaged the dura when the inner drill snaps forward as it cuts through the inner surface of the skull. This is particularly true in older people since the dura is ordinarily closer to the skull. In addition, the inner drill may engage a bone fragment or "petal" as it cuts through the inner surface of the skull and drive it through the dura into the brain, causing serious injury. Such "petals" are particularly dangerous inasmuch as they are small and may have sharp cutting edges. Even though the inner drill may have disengaged on penetration of the skull, the inertia of the drill, turning at speeds of up to 1000 RPM, can impart considerable force to these petals.